Giulia Folpini, Klaus Reimann, Michael Woerner, Thomas Elsaesser, Johannes Hoja, and Alexandre Tkatchenko
https://journals.aps.org/prl/accepted/ae076Yc0Mb51db56829d2d3865649318b071feee8
The nonlinear response of softmode excitations in polycrystalline acetylsalicylic acid (aspirin) is studied with two-dimensional terahertz spectroscopy. We demonstrate that the correlation of CH3 rotational modes with collective oscillations of \pi electrons drives the system into the nonperturbative regime of light-matter interaction, even for a moderate strength of the THz driving field on the order of 50~kV/cm. Nonlinear absorption around 1.1~THz leads to a blueshifted coherent emission at 1.7~THz, revealing the dynamic breakup of the strong electron-phonon correlations. The observed behavior is reproduced by theoretical calculations including dynamic local-field correlations.
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